1. Field of the Invention
This invention relates to an automatic frequency offset compensation apparatus for effecting a frequency offset compensation to a received digital modulated radio signal.
2. Description of the Prior Art
An automatic frequency offset compensation apparatus for performing a frequency offset compensation is known. Such a prior art automatic frequency offset compensation apparatus obtains a vector phase difference between a complex baseband signal derived from a quadrature detection of a received modulated radio waves and a predetermined frame synchronizing signal and performs the frequency offset compensation using the vector phase difference.
FIG. 6 is a block diagram of a prior art automatic frequency offset compensation apparatus. A vector correlation portion 602 generates a vector correlation signal 603 from a quadrature baseband signal 601 and a predetermined vector signal 604. A frequency offset prediction portion 605 renews a prediction value 606 of a frequency offset using the quadrature baseband signal 601 and the predetermined vector signal 604 only when an absolute value of the vector correlation signal 603 exceeds a predetermined a correlation threshold value. A frequency offset compensation portion 607 compensates the quadrature baseband signal 601 using the renewed frequency offset prediction value 606 and outputs a compensated quadrature baseband signal 608.
This prior art automatic frequency compensation apparatus has a problem that if a received signal includes a large amount of distortion due to the multipass phasing or the like, a possibility that the absolute value of the vector correlation signal exceeds the predetermined correlation threshold value is low, so that an interval of the prediction of the frequency offset becomes long.